Continuous heat treat furnaces such as conveyor belt, shaker hearth or shuffle hearth furnaces are often used to heat treat large quantities of relatively small pieces of work such as screws, nuts, washers, bolts and pins. The handling of such small parts particularly during the quenching process has long presented problems in two major areas, one involving the metallurgical requirements for a thorough quench of each and every small piece and the other the practical economic requirements for recovery of the quenched pieces. Installation and maintenance of the quenching apparatus itself as well as the need to minimize both the initial quantity of quench liquid required and the loss of quench liquid during use also present serious problems.
It has long been the practice in the prior art to include pit type quench systems such as those shown in the U.S. Pat. to Krach et al, Nos. 3,531,096, and Keough 3,650,853 for use in connection with furnaces of the conveyor belt, shaker hearth and shuffle hearth varieties. While the disadvantages of such pit type quench systems have been appreciated, to date no suitable alternative has been developed. The principal problem associated with the prior art pit type quench systems is the mechanical means necessary to recover the parts from the quench pit. While it might at first impression seem to be fairly simple to provide a conveyor means such as a mesh belt at the bottom of a quench pit onto which the work pieces would drop after passing through the quench liquid, the environment within which such a conveyor belt must function causes considerable difficulties. Custom design of the pit type quench system to the particular heat treating operation can somewhat reduce the problems that will be experienced; however, such a solution is not available to the job shop type heat treater. Unless enough pieces of a particular type are heat treated to maintain a furnace and pit quench in operation full time, it is often necessary to vary heat treating cycles, the size of the pieces being treated, and the quench medium. Obviously, any such changes which must be made during the operation of a pit type quench present serious and at times expensive problems. The conveying mechanism, usually a mesh conveyor belt, may not function as efficiently at a higher speed which may be necessary to accommodate a short heat treat cycle. A mesh size that works well for one size part may allow a smaller part to drop through the belt. Pit quenches require a considerable volume of quenchant and it involves a considerable expense to change the type of quenchant.
Very often work dropping through the quench liquid in a pit quench does not stay on the conveyor belt. When such parts fall off the belt they can jam the belt or otherwise damage it so as to require maintenance. In order to repair or free such a belt it is necessary to either evacuate the quench liquid from the pit or withdraw the mechanism, neither of which alternatives lend themselves to rapid repair of the down equipment. Maintenance of both the parts handling mechanism and the quenchant itself is made more difficult by the buildup in the pit of scale from the quenched parts.
Splashing of the quenchant is another problem resulting from the work dropping into the quench liquid. The heat transfer from the pieces to the quench liquid can produce boiling at the surface which increases the splashing. Aside from its deleterious effect upon the quality of the quench and the possible damage to the furnace that it can cause, splashing results in a loss of the quenchant. At times, the loss can be quite significant and, coupled with loss of the quenchant resulting from dragout, may require constant and expensive addition of fresh quench liquid.
In addition to problems directly related to the quenching apparatus and the quenchant, the pit type quench systems can also be troublesome from the standpoint of providing a thorough quench of each and every piece of work. As the work is dropped from, say, a shuffle hearth, the pieces can, depending on their particular size or shape, group or lump together into a rather solid mass. Obviously, a relatively solid mass will not have the surface area of each piece comprising that mass properly exposed to the quenching liquid. Hence, the desired metallurgical properties will not be obtained. Even if massing of the work pieces does not occur, unless the quench liquid itself is properly circulated certain zones could become overheated, decreasing the temperature difference between the work and the quench liquid and, hence, decreasing the quality of the quench.
Although there is no teaching in the prior art of a quench system which would solve the problems discussed above, British patent No. 939,453 does disclose, rather schematically, a quench system wherein the quenchant is used to convey the parts as they are being quenched. While the invention herein disclosed also utilizes the quenchant to both quench and convey the parts, there are significant differences in the structure of this system, particularly at the inlet and the discharge, which result in a practical, commercially operable quench system not taught or suggested by the British patent.